Structure and Reactivity of a Manganese(VI) Nitrido Complex Bearing a Tetraamido Macrocyclic Ligand

• Published in: Journal of the American Chemical Society 2021-09, Vol.143 (38), p.15863-15872
• Main Authors: Shi, Huatian, Lee, Hung Kay, Pan, Yi, Lau, Kai-Chung, Yiu, Shek-Man, Lam, William W. Y, Man, Wai-Lun, Lau, Tai-Chu
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.1c08072

Manganese complexes in +6 oxidation state are rare. Although a number of Mn­(VI) nitrido complexes have been generated in solution via one-electron oxidation of the corresponding Mn­(V) nitrido species, they are too unstable to isolate. Herein we report the isolation and the X-ray structure of a Mn­(VI) nitrido complex, [MnVI(N)­(TAML)] – (2), which was obtained by one-electron oxidation of [MnV(N)­(TAML)]2– (1). 2 undergoes N atom transfer to PPh3 and styrenes to give Ph3PNH and aziridines, respectively. A Hammett study for various p-substituted styrenes gives a V-shaped plot; this is rationalized by the ability of 2 to function as either an electrophile or a nucleophile. 2 also undergoes hydride transfer reactions with NADH analogues, such as 10-methyl-9,10-dihydroacridine (AcrH2) and 1-benzyl-1,4-dihydronicotinamide (BNAH). A kinetic isotope effect of 7.3 was obtained when kinetic studies were carried out with AcrH2 and AcrD2. The reaction of 2 with NADH analogues results in the formation of [MnV(N)­(TAML-H+)] – (3), which was characterized by ESI/MS, IR spectroscopy, and X-ray crystallography. These results indicate that this reaction occurs via an initial “separated CPET” (separated concerted proton–electron transfer) mechanism; that is, there is a concerted transfer of 1 e– + 1 H+ from AcrH2 (or BNAH) to 2, in which the electron is transferred to the MnVI center, while the proton is transferred to a carbonyl oxygen of TAML rather than to the nitrido ligand.